1. Field of the Invention
This invention relates to stator cores of dynamoelectric machines, and more particularly, to means for ensuring uniform axial expansion of the stator core when subjected to elevated temperatures.
2. Description of the Prior Art
Stator cores of large dynamoelectric machines such as generators are typically supported within a stationary frame and usually constitute many, axially thin annular laminations which are assembled together and compressed to form a cylindrical stator core. Radial slots formed in the radially inner periphery of the stator core receive stator coils therein which extend beyond both axial ends of the stator core and are suitably interconnected to form a stator winding. The portions of the stator coils which extend axially beyond the ends of the stator constitute end turns which are supported by coil support assemblies. Such coil support assemblies are primarily disposed on the radial outside of the end turns and are connected to stator core support systems. Through-bolts extend axially through the stator core laminations, through magnetic end shields disposed on each end of the stator core, and through the core support system. Building bolts disposed along the radially outer periphery of the stator core laminations similarly extend through the magnetic end shields and core support systems. Nuts disposed on both axial ends of the through-bolts and building bolts have typically been sufficiently torqued to hold the core support systems in the desired, assembled position and maintain the axial continuity and integrity of the stator core.
While the through-bolts are free to slide axially relative to the dynamoelectric machine's frame, the building bolts are usually welded or otherwise securely fastened to bore rings which circumferentially surround and support the stator core at predetermined axial locations. The bore rings, in turn, are rigidly connected to the dynamoelectric machine's frame to prevent relative axial movement between the bore rings and frame. During operation of the dynamoelectric machine, the stator core and previously mentioned bolts are subjected to elevated temperatures primarily resulting from losses and inefficiencies sustained in the stator core. Such elevated temperatures cause the stator core laminations and through bolts to undergo thermal expansion in both axial directions. The building bolts, however, cannot appreciably expand in the axial direction due to their rigid axial interconnection with the machine's frame. Since the building bolts do not significantly expand in the axial direction and the through bolts do, the magnetic end shields, core support systems, and attached coil support assemblies rotate about the nuts connected to the building bolts. Such rotation can result in separations between components of the coil support systems. Such separations permit radially outward movement of the stator winding's end turns and concomitant rubbing, wearing, and fretting of elements such as the coils, coil insulation, coil spacers, and tapered separation blocks disposed between the coils at each end turn. Such wearing and fretting of the various elements can have a deleterious effect on their performance and may, after extended operation time, necessitate repair and/or refurbishment of such elements. Repair of such elements normally requires removing the dynamoelectric machine from service at great cost and inconvenience to the operating utility.
Since the through-bolts typically constitute non-magnetic material which has a rate of thermal expansion greater than that of the stator core laminations, the aforementioned rotation-separation phenomenon is further accentuated. As a result, uniform axial expansion of the stator core has been judged to be necessary to eliminate the rotation phenomenon and avoid the excessive generator element wear.